1. Field Of the Invention
The invention relates to methods and apparatus for continuously monitoring and controlling the composition of flux used in the soldering of printed circuits.
2. Description of the Related Art
In the manufacture of printed circuit boards, a flux is applied to a board surface before the surface is contact coated with solder. Fluxes used in the printed circuit manufacturing industry usually include four components: an activator; a solvent; surfactants; and corrosion inhibitors. Typical activators include rosin acids (10-20%) and dicarboxylic acids, such as adipic acid, usually present in the range 1-2 wt. %. The most commonly used solvents are alcohols. Surfactants are often added at a rate of 0.1-0.2 wt. % to promote foaming of the flux and to act as a wetting agent in applying the foamed flux to printed boards. Corrosion inhibitors are added to reduce corrosion and prevent corrosion of metallic circuitry during the soldering process.
Flux is typically applied to the board by means of a foam head generated by aerating the flux. However, this aeration of the flux causes significant loss of solvent through evaporation. If this evaporation were to proceed unchecked, then the solids concentration of the flux would increase with time in a continuous process unless make-up solvent is added to compensate for solvent losses. Thus, either solvent loss or flux composition must be monitored to determine the quantity of make-up solvent that must be added to maintain a constant flux composition. To accomplish this, the composition of the flux is usually measured on a continuous or semi-continuous basis by sampling the flux and measuring its specific gravity. This technique is useful when there is a relatively large difference in specific gravity between the flux and the solvent. For example, the specific gravity of a typical RMA flux, such as Kester 34T18, is 0.834 while the specific gravity of a typical solvent, such as isopropyl alcohol, is 0.783. Typically, the nonvolatile residue of the flux material is about 18 wt. %.
While flux composition can therefore be relatively easily controlled, currently used rosin-based fluxes are typically cleaned from circuit board surfaces using chlorofluorocarbon (CFC) solvents. However, the United States Environmental Protection Agency (EPA) has determined that CFCs have a detrimental effect on the ozone layer in the upper atmosphere and has restricted the use of these solvents in the short term and will prohibit their use within a few years. As a result, circuit board manufacturers are converting their production operations to eliminate the use of CFC solvents for cleaning by substituting new flux materials which can be cleaned with other solvents, principally water, or which can be left on board surfaces without adversely affecting the long-term reliability of printed circuits.
The new flux materials are typically low-solids fluxes containing only about 2-5 wt. % non-volatile residue. Further, the specific gravity of a typical low-solids flux, such as Kester 951, is about 0.813, while the specific gravity of a typical solvent, such as ethanol, is 0.805. Thus, the difference between the specific gravity of the flux and the solvent is significantly less than for the higher solids rosin-based fluxes. Indeed, the specific gravity differential is too small for precise measurement and control using specific gravity as the controlled variable.
Since relatively simple specific gravity measurements cannot be used with any degree of reliability to control the composition of low solids fluxes that do not require subsequent CFC-cleaning, solvent composition may be monitored directly by gas chromatography. However, although this technique can be used online in the process environment, commercial chromatograph instrumentation is relatively expensive to install and maintain. What is needed is a method for continuously monitoring the solvent content of low solids fluxes so that, as solvent is volatilized through the aeration process, make up solvent can be added to maintain the flux composition at a desired controlled level.